1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer, a facsimile apparatus. More particularly, the invention relates to an image forming apparatus that transfers images on an image bearing member to an intermediate transfer member, and then, transfers the images on the intermediate transfer member to a transfer material.
2. Related Background Art
For the conventional color image forming apparatus (a copying machine or a laser beam printer, for example), there has been known an image forming apparatus of the so-called intermediate transfer type where a plurality of toner images formed by a plurality of image forming sections, respectively, are transferred once to the intermediate transfer member sequentially to superimpose them on the intermediate transfer member, and after that, the images are transferred from the intermediate transfer member to a transfer material collectively.
FIG. 7 is a view which shows one example of the image forming apparatus described above.
As shown in FIG. 7, the apparatus is provided with the four image forming sections Pa, Pb, Pc, and Pd, and the intermediate transfer belt (intermediate transfer member) 71. For each of the image forming sections Pa, Pb, Pc, and Pd, there is provided each of photosensitive drums 101a, 101b, 10c, and 101d; charging members 102a, 102b, 102c, and 102d; exposure devices 103a, 103b, 103c, and 103d; developing devices 104a, 104b, 104c, and 104d; cleaning devices 105a, 105b, 105c, and 105d; and transfer members 106a, 106b, 106c, and 106d.
In each of the image forming sections Pa, Pb, Pc, and Pd, each of the toner images having different colors is formed by each of the members described above. The toner images thus formed are sequentially and primarily transferred to the intermediate transfer belt 71 which is passed over rollers 132 and 133 below the image forming sections Pa, Pb, Pc, and Pd to travel in the direction indicated by an arrow R71. Then, four color toner images are superimposed on top of one another on the intermediate transfer belt 71. The images thus superimposed are secondarily transferred to a transfer material P in the secondary transfer position 111 between the intermediate transfer belt 71 and a transfer roller 72. The transfer material P is supplied from a sheet feed cassette 90, in which the transfer material is stored, to the secondary transfer position 111 by registration rollers 125A and 125B and others at the timing that matches with that of the toner image on the intermediate transfer belt 71. After the transfer of the toner image, the transfer material P is discharged to the outside of the main body of the image forming apparatus by a discharge roller 145 after the toner image is fixed on the surface of the transfer material by a fixing device 140.
Generally, the color image forming apparatus described above is often required to deal with the situation where monochromatic images are output as documents or some others more than color images at offices or the like, for example. Therefore, the apparatus is arranged to be able to selectively output color images (the toner image formed by superimposing the four-color, three-color, or two-color images) and monochromatic images (the monochromatic toner image; mostly black toner images).
However, for the image forming apparatus of the intermediate transfer type as described above, there is a problem in that it takes an unnecessary amount of time to output an image monochromatically.
In other words, when the monochromatic image is outputted, the toner image formed by the photosensitive drum 101d of the image forming section Pd is primarily transferred to the intermediate transfer belt 71 in the primary transfer position 110, and after that, the toner image is conveyed to the secondary transfer position 111 when the intermediate transfer belt 71 is rotated almost half the one revolution in the direction indicated by the arrow R71. As a result, it takes a period of time before the toner image is conveyed to the secondary transfer position 111 from the primary transfer position 110. It takes a period of time inevitably to output an image even monochromatically.
Also, when a monochromatic image is formed by making each of the peripheral speed of the photosensitive drum, the peripheral speed of the intermediate transfer belt, and the peripheral speed of the roller pair in the fixing device faster than that when a color image is formed, there is encountered a problem in making the image forming apparatus smaller at lower costs, because in such a case, a transfer nip should be made larger in a conveying direction of a transfer material or the transfer electric current (voltage) should be made larger in order to secure the transferability, and a fixing nip should be made larger in the conveying direction of the transfer material and the fixing temperature should be set at a temperature still higher in order to secure the fixing capability of the toner image.